An automobile radar system is described in European Patent No. 498 524, for example. This publication describes an automobile radar system using two separate antennas for the transmission and reception paths. The transmitting antenna consists of a horn antenna with an antenna lens arranged in front of it, and it has a relatively wide antenna lobe. The receiving antenna consists of a total of three receiving elements and likewise an antenna lens. It forms three antenna lobes which are offset relative to each other and with which the angles of the received radar signals can be assigned. One problem with the radar system described here is that it needs a relatively large amount of space because it uses two separate antennas for the transmitting and receiving functions. Furthermore, relatively long leads to the antenna elements are needed in such a case, which results in an unwanted power loss because of the attenuation, especially in reception. Another problem with such a radar system is correcting its structure-related asymmetry.
A similar principle of a radar system with separate transmitting and receiving lobes is known from Radar Handbook, by M. Skolnik, 2.sup.nd edition, page 20.7f, for example. This book describes a stacked-beam radar which has several receiving lobes stacked vertically. A possible radar target, however, is targeted over only a single transmitting lobe which is wide enough to cover all the receiving lobes. The statements made above are also applicable with regard to the problems or difficulties associated with such a system.
German Patent Application No. 195 30 065.3 (corresponding to International Patent No. WO 97/02496) describes a multi-beam automobile radar system that uses the same antenna for transmitting and receiving. This antenna consists of a total of three transmitting/receiving elements as well as a common antenna lens arranged in front of them. Thus, it also forms three antenna lobes. The problem with such a radar system using a common antenna for transmitting and receiving is in separating the transmission signal from the reception signal. In the radar system described here, this is accomplished with a circuit arrangement which is referred to below as a double rat race. Such a double rat race is a circuit arrangement that is manufactured preferably by the bus strip technology and connects a duplexer and a receiver mixer. Details of operation are described, for example, in British Patent Application No. GB 2 290 000. With the help of such a double rat race, transmission and reception signals can be transmitted and received over one antenna at the same time. Thus, such a double rat race is especially suitable for a radar system according to the FMCW method. However, such a double rat race also leads to system-related attenuation which is apparent with the naturally weak reception signals of a radar system in particular.